Conductive paste, electroconductive body and fabrication of same

ABSTRACT

A conductive paste and an electroconductive body fabricated from the paste, in which the paste comprises inorganic non-metallic particles coated with silver, silver particles and an organic binder, or inorganic non-metallic particles coated with silver, silver particles, particles of glassy material and an organic vehicle. An electroconductive body comprises silver particles, and inorganic non-metallic particles coated with silver, both embedded in a matrix of organic material or of glassy material. The silver particles and silver-coated inorganic non-metallic particles are in effective contacting relationship within the matrix. Also disclosed are a silver-coated glass bead, for incorporation in paste and an electroconductive body, and methods for fabricating an electroconductive body. The electroconductive bodies are useful, for example, as termination elements for capacitors, and as internal conductive elements in capacitors of the type used in thick-film technology applications.

This is a division of application Ser. No. 187,428, now U.S. Pat. No.4,419,279, filed Sept. 15, 1980.

The present invention relates to conductive paste and toelectroconductive bodies fabricated from such paste, to the making ofthese electroconductive bodies, and to glass beads having a conductivecoating and suitable for making conductive paste and electroconductivebodies.

BACKGROUND OF THE INVENTION

Prior to this invention, conductive paste has been fabricated fromsilver particles, an inorganic bonding component, and an organic bindingcomponent. Typically, such a paste contains, by weight, 60 to 70%silver, 5 to 10% glass frit and 20 to 35% of a mixture of varioussolvents, plasticizers and resins. This paste has been applied to asubstrate, for example, a ceramic capacitor, and the substrate and pastefired to form a component comprising the substrate and a fired-onelectronconductive body which provided an electrically conductiveconnection. Frequently, these components have been coated with solderfor ready integration into a circuit at a later date.

The foregoing prior paste compositions and electroconductive bodies are,in some applications, unsatisfactory because, among other reasons, aftercontact with the solder some of the silver in the electroconductive bodymigrates or leaches out of the body into the solder. This phenomenon isknown in the art as "de-wetting" and often causes a significant decreasein the conductivity of the electroconductive body, as well as impairingits adhesion to the substrate surface.

Another disadvantage of these precursor paste compositions andelectroconductive bodies is that fabrication of the electroconductivebody must be performed by heating the paste at high temperatures whichmay damage the substrate to which a paste has been applied.

Attempts have been made to find satisfactory substitutes for theabove-mentioned pastes and electroconductive bodies. It has beensuggested that a conductive paste may be fired to produce anelectroconductive body containing micro-sized glass spheres coated witha noble metal, such as palladium or platinum, or an alloy, for exampleof palladium, gold and silver, embedded in a matrix of glassy dielectricmaterial having a fusion temperature lower than the softeningtemperature of the glass spheres. An electroconductive body consistingof particles of alumina coated by palladium, particles of alumina coatedby palladium oxide and particles of silver embedded in a glassy matrixhas also been suggested. Although electroconductive bodies of thischaracter may be said to exhibit a somewhat increased resistance tode-wetting, the use of metals such as palladium and gold entailssubstantial expense in the production of the electroconductive body, andits precursor conductive paste. Also, disadvantages attendant tofabrication at high firing temperatures are not avoided with thesesubstitutes.

It has also been suggested that a conductive paste comprising an organicresin binder and a particulated electrically conductive metal-containingmaterial, for example silver-coated glass spheres may be treated to forma conductive coating. Other electrically conductive metals are alsosuggested. However, an electroconductive body with better electrical andother properties, at comparable or lesser expense, would beadvantageous.

SUMMARY

One general object of this invention is to provide a new and improvedpaste or electroconductive body.

More specifically, it is an object of this invention to provide aconductive paste and an electroconductive body comprising relativelyinexpensive materials.

It is also an object of this invention to provide a conductive pastewhich has a consistency appropriate for screening on or otherapplication to a substrate.

It is another object of this invention to provide conductive paste whichis formable into an electroconductive body at conditions not destructiveto an attached substrate.

It is yet another object of this invention to provide anelectroconductive body which is durable and exhibits acceptableconductivity for long periods of time during storage and operation.

It is still another object of this invention to provide anelectroconductive body which, when attached to a substrate, exhibitsacceptable adhesion to such substrate for long periods of time instorage and operation.

It is a further object of this invention to provide an electroconductivebody the performance of which is not impaired by de-wetting when thebody is in contact with solder.

It is a still further object of this invention to provide a method forproducing the foregoing electroconductive body.

It is also an object of this invention to provide a glass bead coatedwith silver which is suited to the production of the foregoingconductive paste and electroconductive body.

In accordance with a feature of the present invention, a conductivepaste comprises inorganic non-metallic particles coated with silver,silver particles and an organic binder formable into a matrix. Thesilver-coated inorganic non-metallic particles and silver particles arein the organic binder thereby forming the paste. The paste is suitablefor application to a substrate, to form an electroconductive body on thesubstrate.

In accordance with another feature of the present invention, aconductive paste comprises inorganic non-metallic particles coated withsilver, silver particles, particles of a glassy material and an organicvehicle. The silver-coated inorganic non-metallic particles, silverparticles and particles of glassy material are in the organic vehicle,thereby forming the paste. This paste is also suitable for applicationto a substrate, to form an electroconductive body on the substrate.

In accordance with another feature of the invention, in severalparticularly advantageous embodiments, an electroconductive bodycomprises silver particles, and inorganic non-metallic particles coatedwith silver, both embedded in a matrix of organic material. The silverparticles and silver-coated inorganic non-metallic particles are ineffective contacting relationship within said matrix.

In accordance with a further feature of the invention, in severalparticularly advantageous embodiments, an electroconductive bodycomprises silver particles, and inorganic non-metallic particles coatedwith silver, both embedded in a matrix of glassy material. The silverparticles and silver-coated inorganic non-metallic particles are ineffective contacting relationship within said matrix.

In another aspect, the present invention relates to a method of makingan electroconductive body, which comprises combining, to form a paste,inorganic non-metallic particles coated with silver, silver particlesand an organic binder formable into a matrix, and subjecting the pasteto conditions sufficient to form the organic binder into a matrix inwhich the silver particles and silver-coated inorganic non-metallicparticles are embedded.

In a further aspect, the present invention relates to a method of makingan electroconductive body, which comprises combining inorganicnon-metallic particles coated with silver, silver particles, particlesof glassy material and an organic vehicle to form a paste, heating thepaste up to a temperature sufficiently high, and for a time sufficientlylong, to fuse the particles of glassy material without deforming thesilver-coated inorganic non-metallic particles, and cooling the product.

A further aspect of the present invention relates to an article ofmanufacture useful in practicing the invention. This article comprises aglass bead which is coated on substantially its entire surface withsilver, the silver constituting at least 25% by weight of the article.

Conductive paste of this invention is useful as an intermediate in themanufacture of an electroconductive body, and more specifically as avehicle by which the components of an electroconductive body areconveniently applied to substrates, such as capacitors, dielectriccomponents, and the like. Electroconductive bodies of this inventionare. in turn, useful to provide an electrically conductive connection orfilm on a substrate. Thus, the electroconductive bodies find applicationas termination elements for ceramic capacitors, such as those of themulti-layer variety. Such electroconductive bodies may also be useful asinternal conductive elements employed in combination with nonconductiveelements in, for example, a multi-layer capacitor or a capacitor of thetype employed in thick-film technology applications. The silver-coatedglass bead of this invention is useful as a component of conductivepaste and electroconductive bodies of this invention, contributing tothe favorable properties thereof and, generally, decreasing productioncost.

The present invention affords the advantages of providing conductivepaste which comprises relatively inexpensive, easily obtainablematerials and which is wellsuited for conversion to an electroconductivebody in a variety of applications. The electroconductive bodies,themselves, are additionally advantageous because of their desirableperformance characteristics, particularly conductivity and ability toadhere to a substrate. In some especially advantageous embodiments ofthe invention the performance characteristics of the electroconductivebodies are not appreciably impaired by de-wetting upon contact withsolder, which is typically applied to a component containing suchelectroconductive body to facilitate its integration into a circuit. Afurther advantage of the present invention is that the silver-coatedglass bead containing at least 25% by weight silver provides aconvenient, relatively low-cost starting material for making anelectroconductive body and precursor conductive paste.

The present invention, as well as further objects and features thereof,will be more fully understood from the following description of certainpreferred embodments, when read with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an enlarged fragmentary sectional view of conductive paste inaccordance with the invention.

FIG. 2 is an enlarged fragmentary sectional view of an electroconductivebody in accordance with the invention.

FIG. 3 is an enlarged fragmentary sectional view of an alternativeembodiment of conductive paste in accordance with the invention.

FIG. 4 is an enlarged fragmentary sectional view of an alternativeembodiment of an electroconductive body in accordance with theinvention.

It will be understood that the views shown in the drawings are not toscale, but that certain aspects, such as amount of organic component,amount of matrix, distances between particles and the like, have beenemphasized for purposes of clarity.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 1 of the drawing, there is shown conductive pastecomprising an organic vehicle 10, illustratively containing polyvinylalcohol, in which are suspended glass beads 12 having a silver coating14, silver particles in the form of flakes 16 and particles 18 of aheat-fusible glassy material. A silver coating 14 on a glass bead 12constitutes, illustratively, 8 to 12% by weight of the bead and coating.

This silver coating on a glass bead serves also to illustrate asilver-coated glass bead containing at least 25%, say 45%, by weightsilver. As typical, the conductive paste is deposited on an appropriatesubstrate 20.

In FIG. 2, there is shown an electroconductive body comprising a matrixof a glassy material 42, in which are embedded glass beads 22, having asilver coating 24, and silver particles in the form of flakes 26. Again,as typical, the electroconductive body is deposited on and adheres to asubstrate 40. A silver coating 24 on a glass bead 22 constitutes,illustratively, 8 to 12% by weight of the bead and coating. Adjacentsilver-coated glass beads are in intersurface contact with one anotherat location 28, or are close enough together, at location 30, so thatelectrons can pass freely between them. Similarly, adjacent silverflakes are in intersurface contact at location 32, or close enoughtogether, at location 34, so that electrons can pass feeely betweenthem. Also, an adjacent silver-coated glass bead and silver flake are indirect intersurface contact at location 36 and, at locations 37, 38, 39,41 are close enough together to allow free passage of electrons. Also,for instance at locations 43 and 44, silver-coated glass beads aresufficiently close to the surface of the electroconductive body to allowfree passage of the electrons between the surface and the silver-coatedbeads. And, for example at location 45, a silver particle issufficiently close to the surface to allow free passage of electronsbetween the surface and the particle. Silver particles and asilver-coated bead are sufficiently close to the substrate, at locations46 and 47 and location 48, respectively, to allow free passage ofelectrons therebetween. Thus, conductive paths through theelectroconductive body are established.

Referring to FIG. 3 of the drawing, there is shown conductive pastecomprising an organic binder 50, in this embodiment comprising amethacrylate polymer, in which are suspended glass beads 52 having asilver coating 54, and silver particles in the form of flakes 56. Asilver coating 54 on a glass bead 52 constitutes, illustratively, 8 to12% by weight of the bead and coating. As typical, the conductive pasteis deposited on an appropriate substrate 60.

In FIG. 4, there is shown an electroconductive body comprising a matrixof an organic material 82, in this embodiment comprising a methacrylatepolymer, in which are embedded glass beads 62, having a silver coating64, and silver particles in the form of flakes 66. Again, as typical,the electroconductive body is deposited on and adheres to a substrate80. A silver coating 64 on a glass bead 62 constitutes illustratively, 8to 12% by weight of the bead and coating. Adjacent silver-coated glassbeads are in intersurface contact with one another at location 68, orare close enough together, at location 70, so that electrons can passfreely between them. Similarly, adjacent silver flakes are inintersurface contact at location 72, or close enough together, atlocation 74, so that electrons can pass freely between them. Also, anadjacent silver-coated glass bead and silver flake are in directintersurface contact at location 76 and, at locations 75, 77, 78 and 79,are close enough together to allow free passage of electrons.Additionally, for instance at locations 83 and 84, silver-coated glassbeads are sufficiently close to the surface of the electroconductivebody to allow free passage of the electrons between the surface and theparticle. Also, for example at location 85, a silver particle issufficiently close to the surface to allow free passage of electronsbetween the surface and the particle. And, silver particles and asilver-coated bead are sufficiently close to the substrate, at locations86 and 87 and location 88, respectively, to allow free passage ofelectrons therebetween. Accordingly, conductive paths through theelectroconductive body are established.

Silver incorporated in a paste in accordance with this inventionprovides a conductive component in an electroconductive body fabricatedfrom the paste. Therefore, the amount and form of the silver in thepaste are in part dependent on the properties desired for such anelectroconductive body.

The coating of silver on inorganic non-metallic particles of theconductive paste is a layer which covers substantially the entiresurface of each such particle. It is preferred that this layer be ofuniform thickness, but thickness may vary from point to point on theparticle surface without departing from the present invention. The layerof silver need only be of sufficient thickness to ensure theconductivity of an electroconductive body produced from the paste of theinvention; however, thickness of the layer may be increased above thisminimum, for instance, to the extent that cost considerations permit.Typically, the thickness of this layer of silver ranges up to 10% of themaximum dimension of the particle. If the inorganic non-metallicparticle is a glass bead, the thickness of the silver coating is, forexample, on the order of 0.1% to 10% of the diameter of the glass beadupon which it is coated.

In accordance with the invention, a preferable silver-coated inorganicnon-metallic particle contains at least 25%, especially at least 40%, byweight silver. It is especially preferable that the particle be a glassbead. Incorporation in the paste of such silver-coated particlesmaintains good conductivity of an electroconductive body made from thepaste without appreciable impairment of its electrical and otherproperties, due to de-wetting, i.e., migration, or leaching out, of somesilver from the electroconductive body. Such de-wetting often resultsfrom contact of the body with solder. It will be understood that costadvantages afforded by the employment of silver-coated inorganicnon-metallic particles as opposed to employment of pure silver alone,are also attendant to this embodiment. Nonetheless, it is also withinthe scope of the present invention to incorporate in a paste, andelectroconductive body made therefrom, silver-coated inorganicnon-metallic particles containing up to 60% by weight silver, forexample, of from 25% to 60% by weight silver. Silver-coated particlescontaining less than 25% by weight silver are also suitable in severaladvantageous embodiments. In a further especially preferable embodiment,the silver-coated particles contain of from 4 to 16% by weight silver.Examples of the latter silver-coated particles are those containingapproximately 4%, 8%, 12% and 16% by weight silver. In accordance with apreferred embodiment, the foregoing applies to glass beads.

The inorganic non-metallic particles, themselves, are suitably irregularin shape, or, alternatively, substantially regular in shape. Thus, theseparticles are, for example, granules, flakes, spheres and spheroids,such as beads. And, in a highly advantageous embodiment of thisinvention, these particles are glass beads which are substantiallyspherical in shape. The silver coating, when deposited on an inorganicnon-metallic particle generally conforms to the shape of the particle,and, therefore, the shape of the silver-coated particle corresponds tothe shape of the uncoated particle. Including the silver layer, thesecoated particles are of a size which is compatible with the attainmentof the desired properties of the conductive paste and electroconductivebody of this invention. That is, the silver-coated particles should besized, for example, so that incorporation of same in the paste does notappreciably interfere with the ease of application of the paste to asubstrate. Typically, the silver-coated particles are of a size from 1to 100 microns in maximum dimension, on average.

In accordance with the foregoing, the silver-coated particles arepreferably silver-coated glass beads which are substantially sphericalin shape. Typically, the coated beads, including the silver layer, areof a size, on average, of up to 75 microns in diameter. Silver-coatedbeads of a size, on average, from 1 to 50, especially 1 to 30, micronsin diameter are preferred. Further examples of appropriately sizedsilver-coated beads are those of a size from 1 to 5 microns in diameteron average, 5 to 10 microns in diameter on average, 10 to 15 microns indiameter on average and 10 to 25 microns in diameter on average. It willbe understood that the foregoing also applies to beads or spheres ofother inorganic non-metallic materials suitable for practicing of thisinvention.

The inorganic non-metallic particles are suitably composed of any of awide range of materials which exhibit properties and physicalcharacteristics consistent with attainment of the objectives of thisinvention. In this connection, it will be understood that "non-metallic"refers to the properties and physical characteristics of thesematerials, and does not preclude the presence of metal atoms or ions aslong as "non-metallic" properties and physical characteristics areexhibited. Suitable materials typically display non-electroconductiveproperties. Accordingly, these materials are, typically, glasses,ceramic substances and naturally occurring mineral substances. Thefollowing are examples of the foregoing materials: oxides, such asbauxite, corundum, ilmenite, brookite, anatase, rutile and magnetite,and hydroxides such as brucite; sulfides, such as galena, pyrite,chalcopyrite and sphalerite; halides, such as sodium chloride, sylviteand fluorite; carbonates such as calcite, magnesite and siderite,nitrates, such as sodium nitrate, and borates, such as borax andkernite; sulfates, chromates and molybdates, examples being celestite,anhydrite and gypsum; and phosphates, such as bivianite, apatite andpyromorphite, arsenates such as erythrite, and vanadates, such asbavanadinite. Additional examples of suitable materials are convenientlyclassified into categories as follows: the tectosilicates, including thesilica group, the feldspar group, the feldspathoid group, the zeolitegroup; the philosilicates, including kaolinite, talc and vermiculite andthe mica group, for instance muscovite, phlogovite and biotite; theinosilicates, including the amphibole group, for instance thecummingtonite series, the pyroxene group, including the hyperstheneseries, for instance spodumene, and the pyroxenoid group; thecyclosilicates including beryl and tourmaline; the sorosilicate group,for instance, idocrase; the neosilicates, including the olivine series,such as magnesium iron silicate, and also including willemite; thealuminum silicate group; the garnet group; and silicates ofindeterminate structure such as prehnite, chrysocolla and dumortierite.It will be understood that synthetic, as well as naturally occurring,inorganic non-metallic materials are suitable for practicing thisinvention. It will further be understood that a preferred shape for thesilver-coated particle is spherical or spheroidal; to spheroidize amaterial often necessitates its heating to fairly high temperatures, andat such temperatures some of the foregoing materials, for example, someamong the hydroxides, sulfides, halides, carbonates, nitrates andsulfates, may be converted to oxides.

In embodiments wherein the silver-coated particles are silver-coatedglass beads, the glass beads, themselves, are typically made of what iscommonly referred to as lime glass, a soda-lime silicate glasscomposition known in the art. However, the beads are also suitably madeof titanium, or other more highly refractory, glass for applicationsrequiring special bead properties, such as thermal resistance, lowalkali content and the like.

In general, the composition of the inorganic non-metallic particlesselected, for example the composition of glass beads, must be such thatthe particles do not soften or appreciably distort in shape underprocessing conditions to which the paste of this invention is subjectedin making an electroconductive body therefrom.

The particles of inorganic non-metallic material are produced in anysuitable or common manner. As an example a suitable method for producingglass beads is to introduce crushed glass particles into a verticallydisposed draft tube. The tube has a source of heat near its lower end,normally a well-distributed gas flame produced by a series of burners.As the combustion gases rise, they proceed into an expansion chamber andcarry with them the glass particles which become soft, so that bysurface tension the particles are shaped into spherical form. The beadsare then cooled and collected. Various bead-producing systems arediscussed in detail, for example, in U.S. Pat. No. 3,279,905, grantedOct. 18, 1966, to Thomas K. Wood et al., U.S. Pat. No. 3,887,914,granted Apr. 15, 1975, to Ib Von Irgens-bergh, U.S. Pat. No. 3,887,918,granted Apr. 15, 1975, to Thomas A. Cerbo, U.S. Pat. No. 3,907,537,granted Sept. 21, 1975, to Ib Von Irgens-bergh, and U.S. Pat. No.4,046,548, granted Sept. 6, 1977, to Thomas K. Wood et al.

Silver-coating of the inorganic non-metallic particles is suitablyeffected by numerous means known in the art. For example, thesilver-coating is applied by fluidization by dry or wet methods, byelectroless plating, and the like. See, for instance, U.S. Pat. No.3,635,824, granted Jan. 18, 1972 to Raymond G. Brandes et al.

As mentioned above, a portion of silver is incorporated in theconductive paste as silver particles. In a preferred embodiment of theinvention, these silver particles comprise flakes of silver. However, itis within the scope of this invention for the silver particles to be ofother shapes. Particularly in cases in which the bead coating has arelatively low silver content, such particles make up at least 5% byweight of the paste. In some embodiments, the silver particlesconstitute at least 10%, or even at least 25%, by weight of the paste.

The total amount of silver incorporated in the paste in the forms ofsilver particles and the coating on inorganic non-metallic particles isat least 10% by weight of the paste. It is preferable to incorporateenough silver in the paste to constitute at least 15%, especially atleast 24%, by weight of the paste. And, it is especially preferable thatthe total silver content be sufficient to prevent appreciable impairmentof properties, such as loss of conductivity and of adhesion to asubstrate, in an electroconductive body (made from the paste) byde-wetting upon contact with solder.

In a highly advantageous embodiment of the invention, the conductivepaste contains an organic binder from which the matrix of organicmaterial of an electroconductive body made from the paste is formed. Insuch body, silver-coated inorganic non-metallic particles and silverparticles are embedded in this matrix. The organic binder suitablycomprises an inert organic material or materials formable into thematrix; the binder imparts to the paste the proper rheology, forinstance, an appropriate consistency for application on a substrate byscreening, painting (e.g., electrostatically or with a brush), dipping(following rack loading), continuous machine dipping, and the like.Typically, the organic binder contains one or more resins and one ormore solvents to give the paste the desired consistency, but in someembodiments the binder is solventless. Examples of suitable substancesare low molecular weight aliphatically unsaturated organic polymers, ora mixture of an aliphatically unsaturated organic polymer and acopolymerizable aliphatically unsaturated organic monomer, such asstyrene. These substances, illustratively, have a viscosity of fromabout 50 to 10,000 centipoises at 25° C. Additional examples are: lowmolecular weight polyimides containing acrylamide unsaturation, forinstance as described in U.S. Pat. No. 3,535,148, granted Oct. 20, 1970to Abraham Ravve; low molecular weight polyesters containing acrylicunsaturation, such as shown in U.S. Pat. No. 3,567,494, granted Mar. 2,1971, to Chester W. Fitko; acrylate esters, and methacrylic esters ofpolyhydric alcohols, for instance as set forth in U.S. Pat. Nos.3,551,246 and 3,551,235, granted Dec. 29, 1970 to Robert W. Bassemir etal. (see also U.S. Pat. No. 3,551,311, granted Dec. 29, 1970 to GeraldI. Nass et al.); acrylate and methacrylate esters of silicone resins;malamine; epoxy resins; allyl ethers of polyhydric alcohols; allylesters of polyfunctional aliphatic and aromatic acids; low molecularweight maleimido substituted aromatic compounds; cinnamic esters ofpolyfunctional alcohols; mixtures of two or more of the foregoing; andthe like. Further examples are unsaturated polymers, such as polyestersfrom glycols and α, β-unsaturated dicarboxylic acids, for instancemaleic and fumaric acids, either with or without other dicarboxylicacids free of α,β-unsaturation, for instance phthalic, isophthalic andsuccinic acids, dissolved in a copolymerizable aliphatically unsaturatedorganic solvent, such as styrene, vinyl toluene, divinyl benzene, methylmethacrylate, or mixtures of such solvents; such systems are set forthin U.S. Pat. No. 2,673,151, granted Mar. 23, 1954 to Howard L. Gerhartand U.S. Pat. No. 3,326,710, granted June 20, 1967 to Mary G. Brodie.Some other examples are unsaturated organosiloxanes of from 5 to 18silicon atoms, and such siloxanes in combination with a vinylic organicmonomer. Illustratively, the organic binder is an acrylic resin or anepoxy resin. Examples of suitable acrylic resins are methacrylatepolymers. Examples of suitable epoxy resins are any monomeric, dimeric,oligomeric or polymeric epoxy material containing one or a plurality ofepoxy functional groups, for instance bisphenol-A and diglycidyl ether.Suitable solvents are coal tar hydrocarbons, chlorinated hydrocarbons,ketones, esters, ether alcohols and ether esters. Examples are xylene,toluene, methylethyl ketone and alcohols, such as aliphatic alcohols ofup to 20 carbon atoms, for instance ethanol and propanol. The binder mayalso contain various common additives such as catalysts and substanceswhich sensitize the binder to radiation, for example, ultravioletradiation. The sensitizers, for example, are suitably incorporated insmall amounts, such as 0.5 to 5% by weight of the binder. Examples areketones, such as benzophenone, acetophenone, and the like, benzoins andsubstituted benzoins, thiourea and aromatic disulfides; also examplesare azides, thioketones and mixtures thereof. The binder is incorporatedin the paste in an amount suitable to impart the above-discussed desiredrheology, for instance in an amount up to 35 to 40% by weight of thepaste, sometimes as low as 15%, and occasionally even down to from 5 to10%, by weight of the paste.

In an alternative, and also advantageous, embodiment of this invention,the conductive paste contains particles of glassy material and anorganic vehicle.

These particles of glassy material are suitably any finely-sizedparticles of vitreous material of the type termed "glass frit".Typically, these particles are of a heat-fusible glassy material. Leadmonosilicate is an example of a low-fusion temperature frit material.The composition and size of particles of glassy material included in aconductive paste are selected based on the criterion that the particlesmust fuse to form a glassy matrix at a temperature below the temperatureat which the silver-coated inorganic non-metallic particles of theconductive paste deform.

The organic vehicle incorporated in this alternative conductive pastesuitably comprises any inert organic material or materials which willimpart to the paste the proper rheology, for instance (and as mentionedpreviously), an appropriate consistency for application on a substrateby screening, painting (e.g., electrostatically or with a brush),dipping (following rack loading), continuous machine dipping, and thelike. The organic vehicle is, for example, an inert organic liquid.Exempary are aliphatic alcohols, for instance of from 2 to 20 carbonatoms such as ethanol or propanol, esters of such alcohols such asacetates and propionates, terpenes such as pine oil, α- and β-terpineoland the like. In an alternative embodiment, the organic vehicle containsnot only a liquid but one or more thickeners, stabilizers, plasticizers,waxes, thermoplastic resins and/or the like. Ethyl cellulose, polyvinylalcohol, resins, for example polymethacrylates of lower alcohol or otherappropriate members of the group described heretofore in connection withthe organic binder, and mixtures of same, are suitable substances foruse in the organic vehicle. The vehicle is incorporated in the paste inan amount suitable to impart the above-discussed desired rheology,generally in an amount up to 35 to 40% by weight of the paste, sometimesas low as 15%, and occasionally even down to or from 5 to 10%, by weightof the paste. The organic vehicle is such that it is substantiallyeliminated from the remaining components of the conductive paste by theconditions to which the paste is subjected during formation of theelectroconductive body; thus, the organic vehicle is not present to anyappreciable degree in an electroconductive body having a glassy matrix.

The conductive paste is made, for example, by combining silver-coatedinorganic non-metallic particles, silver particles and an organic binderformable into a matrix, or silver-coated inorganic non-metallicparticles, silver particles, particles of a glassy material and anorganic vehicle. For example, the silver particles, in admixture withthe organic binder or vehicle, can be wetted in a three-roll mill; thenthe silver-coated inorganic non-metallic particles can be incorporatedand appropriately mixed into the system in a suitable apparatus, forexample, a SPEX mixer or a common paint shaker. The resulting paste, assuch, is then applied to a substrate, for example, a capacitor or aresistor or other dielectric component, etc., in connection with thefabrication of a further circuit component, or is packaged, and storedor shipped for subsequent use.

In accordance with this invention, an electroconductive body isfabricated from a conductive paste comprising silver-coated inorganicnon-metallic particles, silver particles and organic binder bysubjecting the paste to conditions sufficient to form the binder into amatrix in which the silver-coated inorganic non-metallic particles andparticles of silver are embedded. During formation of the matrix, anysolvent component present in the binder is for the most part, preferablycompletely, eliminated.

Typical techniques and conditions for forming the matrix from theorganic binder are: air-drying of the paste at room or elevatedtemperature; heating of the paste up to a temperature of about 350° C.for a time sufficient for matrix formation, ultraviolet irradiation ofthe paste; catalyzed curing of the paste at a temperature within a rangesuitable for operation with the selected catalyst. Other commonlypracticed methods for forming, for instance curing, the organic binderinto a matrix are also suitable. The resultant matrix is an organicmaterial produced by the action of the selected forming technique and/orconditions. Thus, the organic matrix is suitably a material formed froma resin or resins, as previously described, in the organic binder, suchresin or resins being polymerized, cross-linked, or the like to make upthe matrix. It will be understood that the technique and conditionsselected for forming of the matrix are dependent on the type of organicbinder employed and that such technique and conditions should causeformation of a suitable matrix without deforming the silver-coatedinorganic non-metallic particles and silver particles, or otherwisealtering the components of the system, so as to impede performance ofthe electroconductive body.

Alternatively, in accordance with this invention, an electroconductivebody is fabricated from a conductive paste containing silver-coatedinorganic non-metallic particles, silver particles, particles of glassymaterial and an organic vehicle by heating conductive paste of theinvention up to a temperature, and for a time, sufficient to fuse thecomponent particles of glassy material into a matrix in which thesilver-coated inorganic non-metallic particles and particles of silverare embedded, and then cooling the fused body. Typically, the paste isheated up to a temperature in the range of from 450° to 900° C., for,say, about five minutes. If heating up is performed in an atmospherecontaining oxygen, for example, ambient air, heat-up and cool-down ratesare rapid, and illustratively about 130° C. per minute, in order tominimize, or prevent, oxidation of the component silver.

Since conductive paste of this invention is suitable for application toa substrate, such as a ceramic multilayer capacitor, it is within thescope of the invention to form an electroconductive body from the pastedirectly on a substrate to which the paste has been applied. Forexample, a paste is deposited on a substrate and suitably air-dried,heated, irradiated, catalytically cured or fired along with thesubstrate to which it has been applied. It will be understood thatconditions for forming the electroconductive body directly on thesubstrate are the same as those set forth previously for such formation,with the additional consideration that forming techniques or conditionsshould not damage or deform the substrate, or, for that matter thesilver-coated particles. Thus, the conductive paste of the invention istypically deposited on a capacitor as "termination paste" or sandwichedaround a dielectric material and an electroconductive body formed toprovide terminal or internal conductive members of various capacitorcomponents. In this manner, conveniently sized and used capacitors,often termed "chips", are obtained for packaging for later use, forencapsulation in a hermetic package of glass (which, generally, itselfrequires firing) or an organic system, for dipping in solder to providecomponents which are readily integrated into circuits as desired, or forleading (usually by solder-dipping).

An electroconductive body in accordance with this invention comprises amatrix of organic material with silver-coated inorganic non-metallicparticles and silver particles embedded therein. In an alternativeembodiment an electroconductive body in accordance with this inventioncomprises a matrix of glassy material with silver-coated inorganicnon-metallic particles and silver particles embedded therein. Typically,the glassy matrix is fused, that is to say, the product of fusion ofparticles of heat-fusible glassy material, as previously discussed. Inboth of these embodiments, the silver-coated inorganic non-metallicparticles and silver particles are in effective contacting relationshipto define one or more electroconductive paths through the matrix. Forthe purpose of this invention, "effective contacting relationship" meansthat silver-coated particles and silver particles adjacent one anotherare in direct intersurface contact, or close enough so that electronscan pass freely from one to the next. Effective contacting relationshipis established between adjacent silver-coated particles, adjacent silverparticles and/or a silver-coated particle and silver particle adjacentone another. This is illustrated in FIGS. 2 and 4.

The size of the silver-coated inorganic non-metallic particles andsilver particles does not change appreciably, during the fabrication ofthe electroconductive body, from the size of these particles in thepaste. Thus the silver-coated inorganic non-metallic particles areappropriately sized, as previously described, and typically are of asize from 1 to 100 microns in maximum dimension, on average. Ifsilver-coated glass beads (or silver-coated spheres of other materials)are employed, they are typically of a size, on average, of up to 75microns in diameter. Preferably, the coated beads are of a size, onaverage, of from 1 to 50, especially 1 to 30, microns in diameter.Further examples are coated beads of a size, on average, of from 1 to 5,from 5 to 10, from 10 to 15, and from 10 to 25 microns in diameter.

Similarly, the distribution and thickness of the silver layer on theinorganic non-metallic particles, as well as the amount of silver in thelayer, is not appreciably altered in this fabrication.

Therefore, as previously indicated, the thickness of the silver layer onsilver-coated inorganic non-metallic particles in the electroconductivebody is preferably substantially uniform; but it is consistent withpracticing of this invention that the thickness of the silver layer varyover particle surfaces, as long as conductivity of the electroconductivebody is not appreciably impaired. Thickness is typically on the order ofup to 10% of the maximum particle dimension, for instance of from 0.1 to10% of the diameter of the bead.

Typically, at least 30 to 40% of the total weight of theelectroconductive body is made up of silver-coated inorganicnon-metallic particles, and the total amount of silver in theelectroconductive body is suitably at least 10% by weight. In someembodiments it is desirable that the total amount of silver is at least25%, and even at least 40%, by weight of the body. In a furtherembodiment of the invention, the electroconductive body containssilver-coated inorganic non-metallic particles whose silver coatingconstitutes up to 60%, for example of from 25 to 60%, by weight of theparticles and coating. In some embodiments, the silver coatingconstitutes at least 25%, preferably at least 40%, by weight of theparticle, for example a glass bead, and coating. However, anelectroconductive body incorporating silver-coated inorganicnon-metallic particles wherein the silver constitutes less than 25%,illustratively of from 4 to 16%, by weight of the coated particle, isalso suitable, and advantageous, for many applications, and is withinthe scope of this invention. Examples are electroconductive bodiescontaining silver-coated particles wherein silver constitutes 4%, 8%,12% and 16%, respectively, by weight of the coated particle. Inaccordance with a preferred embodiment the foregoing applies tosilver-coated glass beads. It is especially preferable that the totalamount of silver which is incorporated into the electroconductive bodybe sufficient to prevent appreciable impairment of the properties of thebody, such as conductivity and adhesion to a substrate, upon contactwith solder.

It will be understood that the less the amount of silver contributed bythe silver coatings of the inorganic non-metallic particles, the greaterthe amount of silver which is incorporated as silver particles,preferably flakes, to achieve the desired electrical and otherproperties. In this connection, particularly where the amount of silverin the coatings is relatively low the silver particles in theelectroconductive body should constitute at least 5% by weight of thebody; in some embodiments it is preferable that silver particlesconstitute at least 10%, and even at least 25%, by weight of theelectroconductive body. However, it is generally preferred that as muchof the silver as feasible be incorporated in the form of silver coatingon inorganic non-metallic particles, so as to maximize the amount ofsilver available for conducting electrons in the body.

The total amount of silver present in the electroconductive body of thisinvention is selected based on the performance characteristics desiredfor the component in which the electroconductive body is to be used.Thus, the conductivity, adhesion to a selected substrate, solderability,solder-leaching resistance, dissipation factor and the like, which arerequired to the electroconductive body to perform compatibly with othermaterials, should be considered in selecting the total amount of silver.In general, all of the foregoing characteristics will be enhanced byincreasing the total amount of silver in the electroconductive body. Itwill be understood that, even with increased silver content to enhanceone or more properties of an electroconductive body, theelectroconductive body of the claimed invention generally exhibitscomparable or superior properties at lower cost than with a body inwhich the conductive component is solely silver particles.

Another factor to be taken into account in selecting materials for someembodiments of this invention is formation conditions, for instancefiring range, which will be required of the conductive paste inconnection with the fabrication of the electroconductive body. It isoften the case that a relatively high firing temperature will benecessary, for instance, to effect adhesion of the electroconductivebody to a substrate with which it is used. Generally, the temperature atwhich conductive paste can be fired is increased by employing aninorganic non-metallic particle comprising a more refractory materialcapable of withstanding higher temperatures, for example silica,feldspar or bauxite; increasing the particle size, for example of aspherical glass particle, such as a bead, and therefore of thesilver-coated particle, also affords greater resistance to high firingtemperatures.

It is a distinct advantage that conductive paste and electroconductivebodies of this invention comprise, in large part, silver-coatedparticles having cores of inorganic non-metallic material, which is, forexample crystalline, such as rutile, or amorphous, such as glass. Thisis in direct contrast to those prior conductive pastes wherein silver,the conductive component, was present entirely in the form of solidparticles. In such prior conductive pastes silver typically wasincorporated in amounts of from 60 to 80% by weight. Due to theincorporation of the above-mentioned inorganic non-metallic cores asignificant amount of the volume previously occupied by silver is nowtaken up by much less expensive material, thereby affording asignificant cost saving. Nevertheless, despite the substitution ofinorganic non-metallic material for a significant amount of the silveremployed in prior paste compositions, the conductivity of anelectroconductive body made with the paste of the invention is at leastas great as that of an electroconductive body containing 70% by weightsilver, and made from a prior paste. Furthermore, this electroconductivebody exhibits favorable electrical and other properties, such asconductivity, adhesion to a substrate and durability; and, in certainembodiments of the invention, these favorable properties are presentdespite contact of the body with solder. Additionally, the inventionaffords convenience and cost advantages since materials employable inthe conductive paste, electroconductive body and silver-coated inorganicnon-metallic particles, especially silver-coated glass beads, are easilyobtainable and relatively inexpensive. Thus, the objects of theinvention are fulfilled in the provision of an electroconductive bodyexhibiting favorable performance characteristics, which is convenientlyfabricated from available materials of comparatively low cost.

The terms and expressions which have been employed are used as terms ofdescription and not of limitation, and there is no intention in the useof such terms and expressions of excluding any equivalents of thefeatures shown and described or portions thereof, it being recognizedthat various modifications are possible within the scope of theinvention.

What is claimed is:
 1. A conductive paste suitable for forming anelectroconductive body, which comprises inorganic non-metallic particlescoated with silver, said silver-coated inorganic non-metallic particlesbeing of a size, on average, of from 1 to 100 microns in maximumdimension, silver particles and particles of glassy material, saidparticles of glassy material having a fusion silver-coated glass beads,in an inert organic vehicle which is substantially eliminated from theremaining components of the conductive paste during formation of theelectroconductive body, the total amount of silver in said pasteconstituting at least 10% by weight of the paste and the amount ofsilver in said silver particles constituting at least 5% by weight ofthe paste.
 2. A conductive paste suitable for forming anelectroconductive body, which comprises glass beads coated with silver,said silver-coated inorganic non-metallic particles being of a size, onaverage, of from 1 to 100 microns in maximum dimension, silver particlesand particles of glassy material, said particles of glassy materialhaving a fusion temperature lower than the deformation temperature ofthe silver-coated glass beads, in an inert organic vehicle which issubstantially eliminated from the remaining components of the conductivepaste during formation of the electroconductive body, the total amountof silver in said paste constituting at least 10% by weight of the pasteand the amount of silver in said silver particles constituting at least5% by weight of the paste.
 3. A conductive paste suitable for forming anelectroconductive body, which comprises glass beads coated with silver,said silver-coated beads being of a size, on average, of from 1 to 50microns in diameter, silver particles and particles of a heat-fusibleglassy material, said particles of glassy material having a fusiontemperature lower than the deformation temperature of the silver-coatedglass beads, in an inert organic vehicle which is substantiallyeliminated from the remaining components of the conductive paste duringformation of the electroconductive body, the total amount of silver insaid paste constituting at least 10% by weight of the paste and theamount of silver in said silver particles constituting at least 5% byweight of the paste.
 4. A conductive paste as defined in claim 3,wherein the silver coating of said silver-coated glass beads constitutesof from 4 to 16% by weight of the silver-coated beads.
 5. A conductivepaste as defined in claim 3, wherein the organic vehicle contains ethylcellulose, polyvinyl alcohol, a polymethacrylate of a lower alcohol, ora mixture thereof.
 6. A conductive paste suitable for forming anelectroconductive body, which comprises glass beads coated with silver,said silver-coated glass beads being of a size, on average, of from 1 to50 microns in diameter, silver particles and particles of a heat-fusibleglassy material having a fusion temperature lower than the deformationtemperature of the silver-coated glass beads, in an inert organicvehicle which is substantially eliminated from the remaining componentsof the conductive paste during formation of the electroconductive body,the amount of silver in said silver particles constituting at least 25%by weight of the paste.
 7. A conductive paste suitable for applicationto a substrate to form an electroconductive body thereon, whichcomprises inorganic non-metallic particles coated with silver, saidsilver-coated inorganic non-metallic particles being of a size, onaverage, of from 1 to 100 microns in maximum dimension, silver particlesand particles of a heat-fusible glassy material, said particles ofglassy material having a fusion temperature lower than the deformationtemperature of the silver-coated inorganic non-metallic particles, in aninert organic vehicle which is substantially eliminated from theremaining components of the conductive paste during formation of theelectroconductive body, the total amount of silver in said pasteconstituting at least 10% by weight of the paste and the amount ofsilver in said silver particles constituting at least 5% by weight ofthe paste and said total amount of silver being sufficient to preventappreciable impairment of conductivity, or of adhesion to the substrate,of said electroconductive body upon contact of the body with solder. 8.An electroconductive body, which comprises silver particles, andinorganic non-metallic particles coated with silver, said silver-coatedinorganic non-metallic particles being of a size, on average, of from 1to 100 microns in maximum dimension, both embedded in a matrix of glassymaterial, said silver particles and silver-coated inorganic non-metallicparticles being in effective contacting relationship within said matrix,and the total amount of silver in said electroconductive bodyconstituting at least 10% by weight of the body and the amount of silverin said silver particles constituting at least 5% by weight of the body.9. An electroconductive body, which comprises silver particles, andglass beads coated with silver, said silver-coated glass beads being ofa size, on average, of from 1 to 50 microns in diameter, both embeddedin a matrix of glassy material, said silver particles and silver-coatedglass beads being in effective contacting relationship within saidmatrix, and the total amount of silver in said electroconductive bodyconstituting at least 10% by weight of the body and the amount of silverin said silver particles constituting at least 5% by weight of the body.10. An electroconductive body, which comprises glass beads coated withsilver, said silver-coated glass beads being of a size, on average, offrom 1 to 50 microns in diameter, and silver particles, both embedded ina matrix of fused glassy material, said silver-coated glass beads andsilver particles being in effective contacting relationship within saidmatrix, the total amount of silver in said electroconductive body beingat least 10% by weight of the body and the amount of silver in saidsilver particles constituting at least 5% by weight of the body.
 11. Anelectroconductive body as defined in claim 10, wherein the silverparticles are flakes.
 12. An electroconductive body as defined in claim10, wherein the silver coating of said silver-coated glass beadsconstitutes of from 4 to 16% by weight of the silver-coated beads. 13.An electroconductive body, which comprises glass beads coated withsilver, said silver-coated glass beads being of a size, on average, offrom 1 to 50 microns in diameter, and silver particles, both embedded ina matrix of glassy material, said silver-coated glass beads being ineffective contacting relationship within said matrix, the amount ofsilver in said silver particles constituting at least 25% by weight ofthe body.
 14. An electroconductive body, which comprises silverparticles, and inorganic non-metallic particles coated with silver, saidsilver-coated inorganic non-metallic particles being of a size, onaverage, of from 1 to 100 microns in maximum dimension, both embedded ina matrix of glassy material, said silver particles and silver-coatedinorganic non-metallic particles being in effective contactingrelationship within said matrix, and the total amount of silver in saidelectroconductive body constituting at least 10% by weight of the bodyand the amount of silver in said silver particles constituting at least5% by weight of the body and said total amount of silver beingsufficient to prevent appreciable impairment of the body's conductivity,and of its adhesion to a substrate, upon contact of the body withsolder.